Thermally stable composite base metal oxide catalysts

ABSTRACT

A composite base metal oxide catalyst of improved high temperature stability is provided by prereacting a selected base metal oxide catalyst or its precursor compounds with a selected refractory high-surface-area spinel or its precursor compounds to form a crystalline solid solution of the catalyst and the spinel. This solid solution has the properties of good catalytic activity and high temperature structural and surface area stability. An example of such a composite catalyst is CuCr2O4 in solid solution with NiAl2O4.

[ 1 Sept. 9, 1975 THERMALLY STABLE COMPOSITE BASE METAL OXIDE CATALYSTS [75] inventors: Larry E. Campbell, Corning; Robert L. Johnson, Beaver, both of NY.

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: Aug. 20, 1973 [2!] Appl. No.2 389,969

[52] US. Cl 252/465; 252/466 J; 252/468; 252/470; 252/471; 252/474 [51] Int. Cl. ..B01J 23/16; B01J 23/62; 301] 23/82;B01J 23/84 [58] Field of Search 252/465, 466 J, 468, 470, 252/471, 474

[56] References Cited UNITED STATES PATENTS 3,001,929 9/1961 Moy et a1 252/465 X 3,363,023 1/1968 Mooi ct al. 252/465 X 3,595,810 7/1971 Kehl l 252/468 3,626,021 12/1971 Michaels ct a1... 252/465 X 3,787,332 1/1974 Sugier 252/471 X Primary ExaminerPau1 F. Shaver Attorney, Agent, or FirmKees van der Sterre; Clinton S. Janes, .lr.; Clarence R. Patty, Jr.

[ 5 7 ABSTRACT A composite base metal oxide catalyst of improved high temperature stability is provided by prereacting a selected base metal oxide catalyst or its precursor compounds with a selected refractory high-surfacearea spine] or its precursor compounds to form a crystalline solid solution of the catalyst and the spine]. This solid solution has the properties of good catalytic activity and high temperature structural and surface area stability. An example of such a composite cata lyst is CuCr O in solid solution with NiAl O,.

7 Claims, No Drawings THERMALLY STABLE COMPOSITE BASE METAL OXIDE CATALYSTS component oxides of the catalyst and spinel is formed.

' For the purposes of the present description, precursor compounds of a selected catalyst or spinel are compounds which, when heated together in the proper pro- BACKGROUND OF THE INVENTION portions. will react to form the selected catalyst or spi- The use of base metal oxides such as copper chromite nel. Examples of precursor compounds commonly emand cobalt oxide as catalysts for oxidation-reduction, ployed in carrying out the method of the present invenhydrogenation, hydrosulfurization, dehydrogenation tion include the nitrates, carbonates, oxides and hyand other reactions is known. In addition, base metal drous oxides of the metallic components of the selected oxide catalysts such as copper chromite have also been catalysts andspin'els. P proposed for use in high temperature processes such as Base metal oxide catalysts which may be stabilize the treatment of automotive exhaust gases to remove according to this invention include C0 0 CoFe O carbon monoxide, nitrogen oxides, and unburned hy- CoMn O.,, CuCr O CuFe O;, CuMn O ZnCo- O and drocarbons therefrom. CuCo O Stabilization of these catalysts is achieved by Unfortunately, many of the known base metal oxide forming solid solutions thereof with refractory highcatalysts do not demonstrate sufficient stability for use surface-area spinels such as NiAl O MgAl O ZnAlin high temperature processes. Thus, at temperatures in 0 CoAl O CuAl O CdAl O MnAl O NiCr- O excess of about 600C, degradation mechanisms such CoCr O,, and MgCr O r as sintering and interaction between the catalyst and its The resulting thermally stable composite base metal support begin to reduce the catalytic activity of many oxide catalysts may be described as comprising crystalcommonly-used oxides below useful levels. For autoline solid solutions which consist essentially of at least motive applications in particular, where reaction temone base metal oxide catalyst selected from the group peratures may reach 800C. or higher, high temperaconsisting of C0 0 CoFe O 'CoMn O,, CuCr O,', ture stability in the catalyst employed is essential. Even CuFe O CuMni O ZnCo O and CuCo O in solid soslight losses in activity may reduce catalytic efficiency lution with at least one refractory high-surface-area spibelow levels required to enable motor vehicles to meet nel selected from the group consisting of NiAl O,, government regulations. MgAl O ZnAl O CoAl O CuAl O 'CdAl. ,O,,

In the prior art, means such as close compositional MnAl O,, NiCr O CoCr O,, and MgCr. ,O,. control and the use of minor additions of stabilizin oxides have been employed in attempts to stabilize thi ac- DETAILED DESCRIPTION tivity of base metal catalysts at high temperatures. As previously noted, two major forms of degradation However, to our knowledge, none of the commonly of base metal'oxide catalysts are sintering, resulting in used stabilizing procedures are effective to stabilize a a substantial loss of catalytically active surface area, wide variety of base metal oxide catalysts at temperaarid-interaction between the catalyst and its support to tures in the 800900C. temperature applications. form catalytically inactive or less active species.

Accordingly, it is the principal object of the present The amount of degradation from sintering alone can invention to provide a method of thermally stabilizing be substantial as evidenced by the data shown below in base metal oxides, particularly for use as catalysts in Table l relating to the surface area and related catalytic high temperature oxidation-reduction processes. activity of certain base metal oxide catalysts not treated It is a further object to provide novel composite cata- 40 according to the present invention. These catalysts lysts containing active base metal oxides which provide were prepared by coprecipitating the hydrous oxides of both excellent thermal stability and a high level of catathe metallic constituents of the catalysts from nitrate lytic activity for oxidation and reduction processes. solutions of appropriate composition with ammonium Other objects and advantages of the invention will hydroxide, washing drying, and firing to 600C. to conbecome apparent from the following description. vert the hydrous oxide mixtures to the catalysts. The catalysts were subsequently fired to 800 and 900C. SUMMARY OF THE INVENTION for l6 hours in consecutive treatments to determine the We have now discovered a method of stabilizing cereffects of temperature on surface area and activity. tain base metal oxide catalysts which essentially com- Surface areas were determined by nitrogen B.E.T. and prises prereacting these catalysts with certain refraccatalytic activities were determined using a differential tory high-surface-area spinels to provide a composite scanning calorimeter to measure oxidation rate as a material with both good catalytic activity and high temfunction of temperature. The differential scanning calperature stability. The composite material is characterorimeter (D.S.C.) data shown are the temperatures at ized as containing a solid solution of the catalyst in the which the exotherm from the oxidation of known fixed spine], rather than asa simple mixture of spinels. To quantities of n-hexane and carbon monoxide (sepabriefly describe our method, a selected base metal rately determined) are 50% of maximum. Lower 50% oxide catalyst of spinel structure or precursor comconversion temperatures indicate a more active catapounds thereof are reacted with a selected refractory lyst. Surface areas are in meters per gram, and 50% highsurface area spine] or its precursor compounds by, D.S.C. conversion temperatures are in F. heating to a temperature at which a solid solution of the TABLE I Initial Surface Area Surface Area Initial D.S.C. Activity Aftcr Activity After Catalyst Surface After After Activity (5071 XUOfC-lo hrs. 900C-l6 hrs. Composition Area 800C.-l6 hrs. ()C.-l6 hrs. CH,.,/CO CGHH/CO CHM/(O 0 ,0, 1 1 4.5 Y 0.4 4l6/ 495/340 550/460 cucr o, 9.7 1.5 0.7 520/390 590/460 630/510 CuFe O a 0.1 0.1' 510/390 700 000 ZnCo. .0 16 3.3 0.1 440/320 510/370 6 0/4 1 0 TABLE I-continued Initial Surface Area Surface Area Initial D.S.C. Activity After Activity After Catalyst Surface After After Activity (507! 800C.l6 hrs. 900C.-l6 hrs, Composition Area 800C.-I6 hrs. 900C.-l6 hrs. H /CO C H /CO C.;Hi|/CO CuC'oJ), l7 0.3 0.l 400/270 560/370 630/420 An example of the degradation of a base metal oxide catalyst through interaction with its support is the case of CuCr O supported on alumina. After firing for 16 hours at 800C, X-ray examination discloses the formation of a copper aluminate solid solution phase and a Cr O phase, both of which are less active than copper chromite for the oxidation of carbon monoxide and hydrocarbons.

In contrast to the low thermal stability of base metal oxide catalysts as above described, certain arrangements of refractory oxides have and maintain a high surface area at temperatures in the 800-900C. range of present interest. Like the base metal oxides previ ously shown, these refractory high-surface-area materials are of spinel structure; however their catalytic activity for oxidation-reduction reactions in the 200900C. temperature range is negligible.

Examples of such thermally stable high-surface-area spinels and data showing the retention of substantial surface area by these structures even after exposure to elevated temperatures are set forth in Table II below. The spinels were prepared by coprecipitating the hydroxides of the selected metals in the proper proportions from appropriate solutions of the soluble salts of these metals, filtering and/or evaporating to dryness, and firing to 600C. for I hour. Surface areas are given in meters per gram.

TABLE II High-Surface-Area Spinels SURFACE AREAS As After m Hours After l6 Hours Surprisingly, we have found that refractory spinels such as shown above in Table II will react with base metal catalysts of spinel structure such as are shown in Table I to form solid solutions which provide a combination of the desirable properties of each spinel component. Thus, the solid solutions formed demonstrate both good high temperature stability and a high degree of catalytic activity for the oxidation of carbon monoxide and hydrocarbons. In general, the best combination of catalytic activity, high surface area and thermal stability is provided by solid solutions containing about 50 mole percent total of base metal oxide catalyst spinels and 50 mole percent total of refractory high-surfacearea spinels; however, good performance can be extended to compositions containing as little as mole percent or as much as 75 mole percent of the base metal oxide catalyst component.

Catalyst systems which either comprise or consist essentially of solid solutions as above described are termed composite base metal oxide catalysts. The presence in such systems of at least some quantity of a solid solution of a base metal oxide catalyst spine] in a refractory high-surface-area spinel is essential in achieving both catalytic activity and thermal stability according to the invention. However, systems comprising a suitable solid solution in combination with, for example, an excess of a refractory high surface area spinel component, may display a useful degree of activity and stability for certain applications. Nevertheless, systems consisting essentially of the described solid solutions are preferred.

The method by which the described solid solutions are prepared is important. The preferred technique is the one which assures the greatest uniformity in the mixture of solid solution components before reaction by thermal treatment. Thus, where the catalyst component and the spinel component of the solid solution may be directly mixed and reacted, we prefer to form an intimate mixture containing both the precursor compounds of the catalyst and the precursor compounds of the spinel, and then to react this mixture by thermal treatment to form a catalytically active solid solution. Mixtures of base metal nitrates, carbonates or other thermally decomposable salts are suitable for this purpose. Particularly preferred are precursors consisting of the hydrous oxides of the metallic solid solution components. especially when formed by simultaneous precipitation from aqueous solutions of metal salts with ammoniacal precipitating agents such as NH;,vapor or NH OH.

Temperatures and times required to form the desired solid solutions depend on the nature of the mixture being treated. Generally, however, treatment at tem peratures in the range of about 400800C. for times ranging from several minutes to a few hours are suitable. The presence of the desired solid solution crystal phase may be ascertained by conventional X-ray diffraction techniques. Incomplete reaction of the precursor compounds or catalyst-spinel mixtures will result in an X-ray pattern indicating the presence of more than one crystal phase, whereas in general preferred solid solutions produced according to the invention shows only a single crystal phase.

In general, no substantial benefits accrue from the presence of free excess oxides such as CuO, C00, NiO, etc., mixed with the solid solution structure, and it is therefore desirable that the solid solution be essentially free of excess oxides which are not constituents of the spinel structures of either the base metal oxide catalysts or the refractory high-surface-area spinels. However, the presence of such excess free oxides may be toler ated if the stability of the solid solution remains unaffected thereby.

Examples of catalytically active spinel solid solutions prepared according to the invention are shown below in Table III. The first four columns in the Table set forth the catalytically-active spinels treated, the thermal treatment and resulting surface area of the pure materials, and the catalytic activity of each material as initially prepared and after 800 and 900C. treatments. Catalytic activities are expressed in degrees Fahrenheit 5 for 50% conversion of n-hexane and carbon monoxide in accordance with the differential scanning calorimeter method employed in generating the data shown above in Table l. The remaining six columns of data in 6 tures with the formation of relatively inactive aluminachromia solid solutions. Nickel oxide, cuprous chromite and copper chromite are formed in the NiO.CuCrsystem at 900C, and drastic losses in surface area Table III set forth surface area and catalytic activity for and activity are observed. One advantage of using both solid solutions of each catalyst with three different NiO and A1 0 spinel components to form a catalysthigh-surface-area spinels: NiAl O ZnAl O and MgAlspinel solid solution resides in the fact that the phase 0 All solid solutions are equimolar catalyst-spinel separations which cause degradation in simpler systems compositions. The surface area and activity of each are prevented. solid solution is given both as initially prepared and The initial activities of solid solutions produced acafter thermal aging treatmentss in air. The bottom row cording to the invention do not exceed the initial activiof data in Table III sets forth surface area results for the ties of the uncombined catalyst components in all uncombined high-surface-area spinels. cases. However, the thermal stability of the stabilized All of the samples shown in Table III are powders catalyst insures nearly equivalent activity after short prepared by coprecipitating the hydrous oxides of the term high-temperature aging such as described herein, metallic constituents, filtering, washing, drying at and greatly improved long-term stability is expected 1 10C. for 16 hours and firing at 600C. for 1 hour. under high temperature conditions clue to the physical TABLE Ill Thermal Uncombined Catalyst CatalystNiAl CatalysbZnAl O (atalyst-MgAbO Treatment Solid Solution Solid Solution Solid Solution Catalyst Surface 50% D.S.C. Surface 50'71 D.S.C. Surface 50% D.S.C. Surface 50% D.S.C. Com- C. Area C H /C 0 Area C HM/CO Area H H/C() Area ('"H 1/(0 position l6 hrs.) (m /g) (F.) (m /g) (F.) (m Ig) (F.) (m /g) (F.)

CuCr. 600 9.7 520/390 74 510 330 43 480/320 98 530 460 800 1.5 590 460 7.2 530 410 7.0 580/450 23 640 540 900 0.7 630 510 5.9 620 460 3.0 600 490 14 620 520 CuMn. ,O, 600 1 1 420 340 124 410 260 90 360/250 217.4 400 300 800 6 530/430 8.0 510 340 2.6 530 430 9.0 510 430 900 0.4 610 520 a cap. 600 1 1 416/ 74 470/310 67 490/320 52.5 560/410 800 4.5 495/340 450/280 1 1 480/300 27 550/370 900 0.1 550 460 CuFe O 600 8 510 390 54 570 300 61 495/310 71.x 545/380 1100 0.1 700/600 16 590 340 635/470 10.0 630 490 900 65 725 625 211C6 0, 600 16 440/320 60 470 300 43.6 530 340 73.5 610/410 800 3.3 510/370 #60 460 310 12.6 530/360 14.6 570 400 900 0.1 610 410 (6Mn. ,o, 600 12 420/420 132 400 370 646 410 420 60.7 440 490 800 5 550/620 15 510 570 9.9 500 610 22.0 470 650 900 C'QFe O, 600 17.9 590 360 54 570 300 63.3 600 320 74.x 620/380 @100 0.1 720/660 16 590/340 1 1.2 680/500 20.3 (130/380 900 CUCOEO; 600 17 400/270 72 570 390 61 .24 500 290 841 601 1 440 1100 0.3 560 370 0.7 540/370 2.0 520/300 22.0 570 360 900 0.1 630 420 N666 600 200 79 1 800 97 26 91 900 43 l6 7 From the data shown above in Table III the increased stability of the solid solutions as compared to their catalytically active components at temperatures in the 800-900C. range is readily apparent. A typical example of the effectiveness of the method is the system CuCr O -NiAl O NiAl O has relatively high surface area even after thermal treatment at 900C. for 16 hours air, asir, but demonstrates essentially no catalytic activity for the oxidation of n-hexane or carbon monoxide. CuCr O demonstrates good initial activity for the oxidation of both carbon monoxide and n-hexane, but suffers marked losses in both activity and surface area after thermal treatment at 800and 900C. Equimolar solid solutions of CuCr O in NiAl O on the other hand, demonstrate both better initial activity and better high temperature stability than copper chromite alone, despite the dilution effect of NiAl O The beneficial effects of combining high-surface-area spinels such as NiAl O with active base metal catalysts such as CuCr O are not obtained when the spinel components A1 0 and Ni() are employed separately. X-ray data indicates that phase separation occurs in copper chromite-alumina solid solutions at higher temperaand chemical stability of the spinel solid solutions produced.

Preferred stabilized catalysts according to the invention include those consisting essentially of NiAl- O in solid solution with a base metal oxide catalyst of spinel structure selected from the group consisting of C0 0 CoFe- O CuCr O CuFe O CuMn O C uCo O and ZnCo- O Also preferred are catalysts consisting essentially of ZnAl O in solid solution with any of the aforementioned base metal oxide catalysts or CoMn O Mg Al- O in solid solution with CuFe- O is also a preferred catalyst.

The effectiveness of stabilized catalysts produced according to the invention can also be evaluated from bench testing data showing the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides by Catalytic converters employing these catalysts. The converters consist of catalyst-spinel solid solutions despersed on refractory ceramic monolithic support structures of the honeycomb type. Catalyst evaluation involves the determination of converter temperatures at which 20%, 50% and of the carbon monoxide, propylene and nitrogen oxide constitu- 7 ents of a test gas stream are converted to harmless constituents. Alternately, the percent conversion of these constituents at projected operating temperatures (about I. 1 F. under steady-state conditions may be determined.

The test gas employed for these evaluations is a simulated exhaust gas mixture containing l.07( carbon monoxide. 250 ppm. propylene, 1.000 ppm. nitric oxide. 10% water vapor, 0-l .257: oxygen and the balance nitrogen by volume. This mixture is passed through the converter at a space velocity of about 15,000 hrf. Oxygen levels are maintained at about 1.25% for the oxidation evaluations and varied from 0-l .257( for the for the reduction evaluations.

Table IV below gives some oxidation results for several supported catalyst-spinel solid solutions produced according to the invention. The results are given in terms of converter temperature (in degrees Fahrenheit) required for 50% and 80% conversion of propylene and carbon monoxide. Each converter is evaluated after initial firing at 600C. for 16 hours and again after heat treatment at 800C. for 16 hours. The catalysts are applied to the converter by precipitating the mixed hydrous oxides of the metallic catalyst con- 8 are the compositions of the catalysts, the heat treatment employed. and the percent conversions of carbon monoxide. propylene and nitric oxide at l100F. and l5 .000 hr. The converters are prepared according to 5 the procedure employed in making the converters used in generating the data shown above in Table IV.

From the data shown above in Table V the effectiveness catalyst-spinel solid solutions in maintaining high conversion efficiency even after exposure to elevated temperatures is readily apparent.

We claim:

1. A process of stabilizing a base metal oxide catalyst bmm'mh was}? of refractory hlgh'hur' of spinel structure selected from the group consisting of face'arca spmel prevlously deposited on the converter, C0304, COFGZOM coMnzoh cuchoh cuFezoh and thereafter firmgat 600C. for l6 hours to msure CuMn2O4 cucogo4 and ZnCO2O4 against high temper fmmmon the dcblred nd summons ature degradation which comprises reacting said cata- TABLE IV Thermal Propylene Conversion CO Conversion Catalyst-Spine] Treatment 'l'emperatures 'l'emperatures (F.) Solid Solution C.-hrs. 20% sow 20% 50W.

NiAl. 1400-15 550 (120 725 3x5 505 (m0 (ul-e. 600-15 535 5x5 055 3:40 435 525 NiAl. sou-1h 15 685 7x5 490 590 070 NiAI O. 800-46 (530 (1145 s45 425 470 5x5 Cuco o 600- 1 o 555 (m0 740 365 440 515 NiAl O. s00-1s 55 735 H35 435 495 590 NiALO. 1400-10 5 15 (140 715 405 450 550 (.oFe. 000-15 665 735 7 10 735 see NiAl. ,O, 140045 715 790 905 510 5x5 770 c'utr o 500-15 550 725 300 395 4x0 NiALO, sou-m 710 s00 370 490 (uMn O (00 1 (7 4x5 525 0x0 370 445 (1x0 7.11A1. .O, sou-1c 590 735 s40 515 (150 x40 ZnALO, #400 10 7x5 055 5(15 705 920 Zn/x1 0. (400 10 755 510 500 565 700 t'uco o 600- 1 (1 525 (115 740 400 445 (105 ZnAl O. 1400 15 705 900 1005 545 05 895 ZnAl. s00-1s 700 775 830 510 545 (150 21ml). 1400-1 (1 (75 720 K15 475 525 (100 ((1Ft .0,- 600-l (100 55 730 555 (595 885 The activity of similarly prepared catalytic converters may also be evaluated from conversion efficiency data under steady-state operating conditions. For these evaluations, the percent conversion of propylene, carlyst prior to use with a refractory high-surface-area spinel selected from the group consisting of NiAl O MgAl O ZnAl- ,O CoAl O CuAl- O CdAl O,. MnAl. O,. NiCr O, CoCr O and MgCr- O to provide a stable composite catalyst material comprising a solid solution which consists essentially of said base metal oxide catalyst in solid solution with said refractory high-surface-area spinel.

2. A process according to claim 1 wherein said cata- 65 lyst is reacted with said highsurface-area spinel by heating a mixture consisting essentially of said catalyst or precursor compounds thereof and said high-surfacearea spinal or precursor compounds thereof at a tem- 9 perature in the range of about 4()0-8()()C. for a time sufficient to achieve some solid solution of said catalyst in said high-surface-area spinel.

3. A process according to claim 2 wherein said mixture consists essentially of precursor compounds selected from the group consisting of the hydrous oxides of the metallic components of said catalyst and said high-surface-area spinel.

4. A thermally stable composite catalyst comprising a solid solution which consists essentially of at least one base metal oxide catalyst of spinel structure in solid solution with at least one refractory high-surface-area spinel, said base metal oxide catalyst being selected from the group consisting of C0 0 CoFe- O CoMn O CuCr O CuFe O CuMn O CuCo O and ZnCo O and said refractory high-surface-area spinel being se- 10 lected from the group consisting of NiAl O MgAl O ZnAl204, COA1204, CuAl204, CdAl-ZO... MnAl O 

1. A PROCESS OF STABILIZING A BASE METAL OXIDE CATALYST OF SPINEL STRUCTURE SELECTED FROM THE GROUP CONSISTING OF CO3O4 COFE2O4, COMN2O4, CUCR2O4, CUFE2O4, CUMN2O4, CUCO2O4 AND ZNCO2O4 AGAINST HIGH TEMPERATURE DEGRADATION WHICH COMPRISES REACTING SAID CATALYST PRIOR TO USE WITH A REFRACTORY HIGH-SURFACE-AREA SPINEL SELECTED FROM THE GROUP CONSISTING OF NIAL2O4, MGAL2O4, ZNAL2O4, COAL2O4, CUAL2O4, CDAL2O4, MNAL2O4, NICR2O4, COCR2O4 AND MGCR2O4 TO PROVIDE STABLE COMPOSITE CATALYST MATERIAL COMPRISING A SOLID SOLUTION WHICH CONSISTS ESSENTIALLY OF SAID BASE METAL OXIDE CATALYST IN SOLID SOLUTION WITH SAID REFRACTORY HIGH-SURFACE-AREA SPINEL.
 2. A process according to claim 1 wherein said catalyst is reacted with said high-surface-area spinel by heating a mixture consisting essentially of said catalyst or precursor compounds thereof and said high-surface-area spinal or precursor compounds thereof at a temperature in the range of about 400*-800*C. for a time sufficient to achieve some solid solution of said catalyst in said high-surface-area spinel.
 3. A process according to claim 2 wherein said mixture consists essentially of precursor compounds selected from the group consisting of the hydrous oxides of the metallic components of said catalyst and said high-surface-area spinel.
 4. A THERMALLY STABLE COMPOSITE CATALYST COMPRISING A SOLID SOLUTION WHICH CONSISTS ESSENTIALLY OF AT LEAST ONE BASE METAL OXIDE CATALYST OF SPINEL STRUCTURE IN SOLID SOLUTION WITH AT LEA ST ONE REFRACTORY HIGH-SURFACE-AREA SPINEL, SAID BASE METAL OXIDE CATALYST BEING SELECTED FROM THE GROUP CONSISTING OF CO3O4, COFE2O4, COMN2O4, CUCR2O4, CUFE2O4, CUMN2O4, CUCO2O4, AND ZNCO2O4, AND SAID REFRACTORY HIGH-SURFACE-AREA SPINEL BEING SELECTED FROM THE GROUP CONSISTING OF NIAL2O4, MGAL2O4, ZNAL2O4, COAL2O4, CUAL2O4, CDAL2O4, MNAL2O4, NICR2O4, COCR2O4, AND MGCR2O4.
 5. A thermally stable composite catalyst according to claim 4 wherein the refractory high-surface-area spinel is ZnAl2O4.
 6. A thermally stable composite catalyst according to claim 4 wherein the refractory high-surface-area spinel is NiAl2O4, and wherein the base metal oxide catalyst is selected from the group consisting of Co3O4, CoFe2O4, CuCr2O4, CuFe2O4, CuMn2O4, CuCo2O4 and ZnCo2O4.
 7. A thermally stable composite catalyst according to claim 4 wherein the refractory high-surface-area spinel is Mg2Al2O4 and the base metal oxide catalyst is CuFe2O4. 